Piezo-electric crystal mounting



Oct. 18, 1932. s. M. THURSTON 1,883,111

PIEZQELECTRIC CRYSTAL MOUNTING Filed Aug. 14, 1929 /Nl/ENTU/-? 5. MTHuRsmA/ A TTUHNEX Patented Oct. 18,1932

' UNITED STATES PATENT [OFFICE GEORGE E. THURSTON, OI NEW YORK, N. Y.,ASSTGNOBTOBELL TELEPHONE LABORA- TORIE S, INCORPORATED, OI NEW YORK, N.Y., A. CORPORATION 01 NEW YORK PIEZO-ELEC'IBIO, CRYSTAL MOUNTING-Applicationflled August 14, 1929. Serial No. 3853M.

This invention relates to the mounting of piezo-electric crystal bodies.5 This invention aims to provide a simple, rugged, easily portable,physically stable, 5' mounting for crystal bodies. Since frequencystability is concomitant with physical stability it isa subsidiaryobject of the invention to provide a mounting for crystal bodiescharacterized by comparatively great frequency stability underconditions where relative movement of the crystal and its immediatelyassociated structure would tend to promote frequency instability.

The frequency bodies is, in addition to other things, a function of thecoupling between the vibrating body and the electric circuit. Thiscoupling in turn is a function of the mounting of the crystal body, thatis, its position with respect to the electrodes by which connection tothe electric circuit is obtained,

In the conventional holders of the prior art in which the crystalbodyxis confined loosely between the electrodes there is a suf ficientmovement, at the best, to account for frequency changes of the order of500 cycles per million when the holder is shaken to the extent whichwould naturally result from use or even from transportation prior toinpractical purposes a plate-like portion of the natural crystal whosethickness is in general an inverse function of the natural frequency,would be used) in the high frequency, that 35 is, short wave, field thatis now becoming popular, this variation would be detrimental .toeffective use. y

I In order-to fix the position of the crystal body or plate relative tnthe electrodes, resort has been 'had to a rigid clamping of the plate ata nodal portion. This has been found practicable where-the plate has aconformation of the relativelyilong bar in which simple longitudinalvibrations in the direction of its length are set up by electrodesplaced on its principal faces. A bar vibrating in this manner has quitedefinitenodalpoints or lines intermediate its boundaries at which rigidclamping'is entirely successful, that is,

, is not attended by excessive damping.

of vibration of crystal stallation. In the use of crystal bodies (for Inthe use of crystal plates generally adapted for short wave applicationswhose frequency is principally determined by the 7 ranged to clamp theplateat the center have been tried without much success on account ofmechanical difficulties in so supporting the platein a fixed position.

Applicant has discovered that along the edges of a rectangular crystalplate normal to the optical axis (the plate being cut, as

- usual, so that it is parallel to the optical axis) although there isnothing that can accurately be described as a node, there is at leastcomparative qiiiescence. Rigid clamping at these parts of the plate hasbeen found effective in stabilizing the frequency without materialsacrifice of activity. The frequency has been found to change onlyslightly on accountof the clamping itself.

The above method of clamping has been found effective for high or lowfrequency plates. For manufacturing r asons circular crystal plates havebeen found more practicable for high frequencies, that is frequencies ofthe order of 1,000,000 cycles per second and up. It has been found thatcrystal plates so cut may be successfully clamped around the periphery,the operating conditions being much the same as in the case ofedge-clamped rectangular plates.

The invention will be more fully understood by reference to thefollowing detailed description and claims when taken in connection withthe accompanying drawingm which:

Fig. 1 represents in plan a crystal plate mounting of the invention, ofthe specific form in which the plate is circular and 1s clamped at theperiphery, the holder being to expose the interior And Figs. 3 and 4 areplan and elevation views, partly broken away, of an alternative type ofcrystal plate-mountingin which a v rectangular plate is clamped atopposite ed es. Iteferring to Figs. 1 and 2 the metalbase plate 1 andcover portion 2 of insulating material are rigidly secured together byscrew 'fastening'means 3 to provide a protective housing for thecirculatory cut piezoelectric crystal plate which is stimulated intovibration, as in accordance with now well knownprinciples, by electrodespositioned at opposite faces so as to impose an electrostatic stressthereon. The electrodes, in the specific form of the device hereillustrated are constituted by the-base plate 1, on the one hand, and bythe plate 5 on the other hand. These electrode members are of course ofconductive material. The crystal plate may conform to any of theconventional =types so far as concerns its relations to the natural axesor dimensions of the natural crystal from which it is cut.: As in allconventional crystals the cutting is such. that the planes oftheprincipah faces, that is, elec trode faces,are parallel with the opticalaxis of the natural crystal. A circularly cut crystal is illustrated indeference to the fact that for very high frequency-,as is hereprincipally contemplated, it is sometimes simpler from a manufacturingstandpoint to cut the crystal in that form, as compared, for ex-- ample,with the perhaps more usual rectangularly cut crystal plate. The baseelectrode may be connected to the circuit with which the crystal plateis associated by a conductor not shown; The upper plate 5 may beconnected to such circuit through spring retainer member 6' and itsscrew fastening means represented generally by. reference 7.

In a practical embodiment of this modification of the invention thecover element 2 is constituted by a material known as Isolantite. Thelining member 8 may be used for the purpose of insuring a close fit between the electrode plate 5 and the cover member. If the cover memberwere constituted by a material subject to working to make it easilyconform to specified size andshape this lining member could be dispensedI with. Inthe practical case it is constituted by a fiber bushing turneddown on the inside to make an accurate fit with the electrode. It shouldbe noted that the two electrodes engage the crystal plate only at theperiphery thereof since they are dished in the center so as to leave arelatively narrow and short ri-m. Furthermore, the crystal plate isrigidly clamped between these electrodes at this portion, the dimensionsand restoring force 1,ses,111

fastenings 3, the upper electrode 5 is pressed on tothe crystal plate,which abuts the lower electrode, with considerable-fore- It has beenfound that if the clamping is made sufficiently rigid to insure anabsolute absence of relative movement of the crystal plate and itsimmediately associated structure under the stress of operation or ,oftransportation and handling, the frequency, willremain stable and therigid clamping will not be attended by excessive damping, that is, lossof activity of the crystal plate. Still further the actual clamping ofthe crystal re-.'

sults in an unsubstantial change of frequency.

Slight adjustment of the frequency may be made bychanges in the clampingpressure and/or by shortening of the clamping rim of either electrode soas to change the spacing between the recessed portion of the electrodeand the opposed crystal surface. The narrownessflof the rim .is notcritical although for best operation the rims shouldclamp the crystalplate over as small an area a's'possiblewithout danger of crushing theedge of the plate. In 'the practical case above noted it wasapproximately one'millimeter wide. 4

- Figs. 3 and 4 represent, considerably. more diagrammatically than inthe instance of the modificatitin in Figs;1 and 2, anadaptation of the,generic principle of' the invention to the case of a clamped rectangularcrystal plate.- The crystal plates are clamped at the parallel edgesnormal to the optical axis. This is achieved by providing each of theelectrodes 1 and 5 with salient portions which correspond to theperipheral clamping rim of the structure of Figs. 1 and 2. The salientportions on each electrode project from one face of the electrodeso-that in the assemblage they project inwardly toward the oppositeelectrode. These projecting portions are spaced apart on each electrodea distance substantially equal to but slightly less than the dimensionof the crystal plate in the direction of its optical axis. This enablesthem to engage the marginal portions of the crystal plate and at thesame time tokeep the .electrode out of direct contact with the majorportion of the area; ofthe crystal plate. Except as required by themodification of structure on account of the different shape of crystalplates the crystal'plate holder assembly may be the same as isillustrated in Figs. 1 and 2 although, for simplicity, slightdifferences are to" be noted in the illustration. The identifications ofthe crystalplate and electrodes, the elements which illustrate theprinciples of the invention, are the same as those used in thesefigures. In view of the above pointed out situation no further explanation of this modification is required.

What is claimed is:

1. In combination a crystal plate having circular electrode faces, aholder therefor comprising a conductive base portion functloning as oneelectrode'and a cover member of insulating material, an electrode withinsaid holder engaging the plate oppositely to the base portion of theholder, and means between said electrode and the inside top of saidcover member cooperating with said base portion to substantiallyimmovably clamp said plate between said electrode and said base portion,said electrode and base portion each having a raised rim-like portionadapted to engage said plate and clamp it at the periphery thereof.

2. A piezo-electric crystal body having opposite faces, electrodestherefor and means including said electrodes for clamping said body tosaid electrodes along the entire margin of said faces and only at themarginal portions of said faces.

3. In combination, a flat plate of quartz having the planes of its flatfaces parallel to the optical axis of the quartz, two similarly shapedelectrode plates between which said quartz plate is positioned, at leastone of said plates having portions projecting from a face extendingtoward the face of the adjacent plate and means clamping said platestightly and holding them rigidly positioned along said projectingportions whereby the remainders of the ad acent faces are held out ofcontact with each other.

4. In combination, a plate of quartz cut with its face planes parallelto the optical axis of the quartz, similarly shaped flat electrodesadapted to be superposed each on one face of said quartz plate, meansengaging opposite marginal portions of said quartz plate to' maintainsaid plate spaced from the flat electrodes over the principal portion ofits area, and means clamping the superposed plates along said oppositemargins in a rigid assemblage.

5. In combination, a quartz plate, the plane faces of which are parallelto the principal axis of the quartz, electrodes therefor, each of saidelectrodes having salient said salient portions being spaced a art adistance substantially equal to and s ightly less than the dimension ofsaid plate'in the direction of its optical axis, and means for clampingsaid plate between said electrodes with the salient portions of saidelectrodes engaging said plate at margins of the plate which aresubstantially normal to the optical axis. a

In witness whereof, I hereunto subscribe my name this 9th day of August,1929.

GEORGE M. THURSTON.

ortions' pro ect1ng above one of. its electro e faces,

